In general, this invention relates to a tolerant ceramic composite and to a process of preparing ceramic composites having enhanced flaw tolerance.
The brittleness and unreliability of ceramics remain difficult problems. Attempts to improve the flaw tolerance of ceramics by imparting “graceful failure,” analogous to ductility in metals, have been partially successful with the use of ceramic composites. Certain ceramic composites have been made less brittle by allowing debonding at an interface (or within an interphase) between a ceramic matrix and a reinforcement, or between layers in ceramic laminates. Laminated composites such as silicon carbide (SiC)/graphite (C) and fibrous monoliths such as silicon carbide (SiC)/graphite (C) and silicon nitride (Si3N4)/boron nitride (BN) can be made less brittle by incorporating a weak interphase and a crack deflecting region. Other oxide systems based on monazite (LaPO4) and the related xenotime (YPO4) interphases have also emerged. More recently, transformation weakened interphases resulting from a stress-induced negative volume change in the ceramic oxide crystal structure in the interphases have been demonstrated to be a viable interphase debonding mechanism.
Porous ceramic composites have been prepared in an attempt to increase the flaw tolerance of ceramics. However, use of these porous ceramic composites as a means for enhancing the tolerance has presented several problems. For example, the pores eventually close from the continued sintering that occurs during prolonged operation at high temperatures. Porous mullite-alumina matrices reinforced with uncoated alumina fibers also show promising results for porous composites, which have relatively simple fabrication requirements. However, the extensive matrix porosity limits the overall mechanical strength of the material, and hence, its use in load-bearing applications.
Thus, there continues to be a need for tough flaw tolerant ceramic components and or processes of providing flaw tolerant ceramic components. This present invention addresses these needs and the above-described problems in a new and non-obvious way and provides a wide variety of additional benefits and advantages.